In a continuous annealing furnace, a process of continuously heating, soaking, cooling and, if necessary, over ageing a steel strip is performed. To provide a steel strip with desired characteristics, not only heating temperature and soaking time, but also uniform quenching of the steel strip is important. Development of high tensile steel used as an automobile material has progressed in recent years. To realize desired tensile strength, flexural characteristics, and elongation characteristics, processes for quenching a steel strip from an annealing temperature in the range of 900 to 800° C. to a temperature in the range of about 300 to 150° C. have been developed.
Various coolants are used to cool a steel strip. Depending on the selection of the coolant, the cooling speed of the steel strip varies.
Cooling can be performed at a high speed by using water as the coolant. However, the biggest problem with water cooling is that the shape of the steel strip changes due to quenching distortion. Moreover, because an oxide film is generated on the surface of the steel strip due to contact with water, it is necessary to provide equipment for removing the oxide film so that high economic efficiency and high productivity are unachievable.
There is a roll cooling method in which a coolant such as water is passed through a roller and a steel strip is cooled by making the steel strip contact a surface of the roller that has been cooled. With this method, a part of the steel strip does not contact the cooling roller when the steel strip is made to contact the cooling roller. Therefore, the steel strip may not be uniformly cooled in the width direction, and operational problems such as meandering and quality problems such as non-uniform quality frequently occur.
As another example, a cooling method using a gas as the coolant is also used. Although the cooling speed of this method is lower than those of the water cooling method and the roll cooling method described above, this method enables more uniform cooling in the width direction of a steel strip. To increase the cooling speed, which is the biggest problem with the gas cooling method, Japanese Unexamined Patent Application Publication No. 2005-146373 discloses an apparatus including a box-shaped header and long gas discharge nozzles attached to the header. The ends of the nozzles are disposed as close as possible to a steel strip, so that the heat transfer coefficient and the cooling speed are increased. Japanese Unexamined Patent Application Publication No. 2006-144104 discloses an apparatus that increases the cooling efficiency by using hydrogen.
However, the conventional technologies described have problems.
In the method described in JP '373, after the gas has been discharged from the nozzles, the gas flows along the steel strip or flows back toward the header. Retention of the gas easily occurs due to the presence of the box-shaped header. Therefore, the temperature of the furnace gas easily increases so that desired cooling performance is not obtained. It has been found that the higher the pressure of discharged gas is, the larger the influence of this phenomenon is. Moreover, there is a problem in that the temperature of the box-shaped header easily rises due to radiant heat received from the steel strip and thereby the total cooling performance is decreased. Furthermore, pressure loss is large because the lengths of the protruding nozzles are in the range of 150 to 200 mm, which are large. Therefore, it is necessary to use a powerful blower so that the power consumption is high, which is not preferable in terms of operation cost.
In the method of JP '104, a gas having a uniform hydrogen concentration (in the range of 20 to 80%) is introduced into the header. Because the temperature distribution in the width direction of the steel strip is not uniform due to influences of gas flow, a furnace wall, the header structure, and the like, the temperature distribution in the width direction does not become uniform if cooling is performed in the same manner in the width direction of the steel strip, and thereby the quality of the steel strip becomes non-uniform. Moreover, because fluttering of the steel strip occurs, the speed at which gas is discharged is limited (within the range of 100 to 190 m/s).
It could therefore be helpful to provide a gas-jet cooling apparatus for a continuous annealing furnace that reduces non-uniformity in the temperature distribution in the width direction of a steel strip, reduces fluttering of the steel strip when the gas is discharged at a high speed, and thereby realizes efficient cooling.